Your png.h seems fine.
There seems to be something wrong in your system in which it thinks
gzseek64(void*, off_t, int)
and
gzseek64(void*, off64_t, int)
are the same function.
Sincerely i don't know what's the issue.

Because there is no poppler-image.cpp in 0.14.
BTW i've changed the blocker to normal, that is there for us developers to use not for people reporting bugs. This might be a blocker for you, but is in no way a blocker for us since for the moment building poppler works for everyone except you so.

I am using the same version of 'libpng' and of 'zlib' in a whole bunch of other targets - everything builds fine.
The two conflicting functions:
gzseek64(void*, off_t, int);
gzseek64(void*, off64_t, int);
can look the same to compiler if somehow 'off_t' and 'off64_t' are set to be the same type. Any ideas how this can happen ?

Sorry, but i don't plan adding a check to see if the zlib version is broken. If you can prove this is the fault in our code i'll fix it. Also if you want to write that configure check, patches are welcome.